Many electrical devices include windings or coils, formed by one or more turns of an electrical conductor. For such windings to operate efficiently, adjacent turns of the windings must be electrically insulated from one another, and from any electrically conducted support structures or cores. Generally, they must also be insulated from the environment to prevent arcing or short circuits caused by environmental factors such as low ambient pressure, or the presence of foreign matter or moisture.
The requisite insulation is typically provided by forming the winding from a length of pre-insulated wire having an outer coating of electrically insulative material deposited upon an electrical conductor. The windings are generally formed by bending a length of wire into a predetermined shape about a mandrel. Alternatively, the winding may be formed by bending the wire directly about a support structure or a magnetic core that is part of the electrical device. Other non-electrically conducting materials in the form of separator sheets or potting compounds may also be utilized in conjunction with the insulated wire to complete the electrical insulation system by providing additional protection against electrical short circuits or environmental factors.
Desirable coating materials for such pre-insulated wire will typically have a high dielectric strength so that only a minimal thickness of insulation is required. A thin coating allows the physical size of the winding to be minimized. Alternatively, if the coating is very thin with respect to the size of the electrical conductor, a larger electrical conductor, or a greater number of turns may be accommodated in the area available for the winding. In general, the coating must not be more than a few thousandths of an inch thick to achieve a power density in the winding that is high enough for most practical electrical devices. The coating must be essentially impervious to the operational environment. The coating must also be flexible enough to allow the winding to be bent into the desired shape without cracking, and yet be tough enough to resist deformation or other damage during fabrication and operation of the winding.
For electrical devices which are not exposed to high temperatures, a variety of organic polymer compounds are suitable for use as electrically insulative coatings. Generally, organic polymer coated wire is formed by drawing the electrical conductor through a die, applying the polymeric coating, and drying or curing the coating to form the finished insulated wire. The resulting coating is adherent, tough, continuous, and can be readily bent without cracking.
Organic polymer coatings have not been suitable for use in electrical devices exposed to elevated temperatures in excess of about 350.degree. F., however, because virtually all known polymers oxidize, degrade, and lose their dielectric properties when exposed to such temperatures. For elevated temperatures in excess of 350.degree. F. other insulation systems must be employed. In the past, two basic approaches have been utilized for high temperature electrical insulation systems, but neither approach has been entirely satisfactory. In the first approach, the organic polymer coating on the pre-insulated wire is entirely replaced with an inorganic coating material, such as glass or a ceramic, that is capable of operating as a dielectric at very high temperatures. The second basic approach utilizes a combination of organic and inorganic materials to form the insulative coating.
The main problem encountered in prior attempts to completely replace the organic coating with an inorganic material has been that all known inorganic glass or ceramic materials are brittle and tend to crack when the wire is bent to form the windings. These cracks tend to significantly reduce the dielectric properties of the coating, and expose the electrical conductor to contamination by moisture or foreign matter in the operating environment. Such contamination increases the likelihood of short circuits or arcing from the winding. The arcing problem is particularly significant for electrical devices operating at high voltage and while exposed to rarified atmospheric environments such as those encountered by aircraft while operating at high altitudes. Electrical performance can thus be severely degraded by such cracking of the coating.
Since brittleness is an inherent characteristic of all inorganic dielectrics, the use of inorganic insulating materials as the sole insulation for electromagnetic windings has been essentially restricted to windings having very large bend radii in comparison to the diameter of the electrical conductor. For wires having diameters larger than about 0.080 in., cracking sometimes occurs even when the bend radii is more than ten times the diameter of the electrical conductor.
In an attempt to provide a wire with an inorganic coating which would withstand bending, some prior electrical insulation systems have utilized a flexible woven sleeve of glass or ceramic material, either directly over a bare electrical conductor or as a secondary insulative layer over a continuous primary insulative layer of either organic or inorganic material. This approach is not entirely satisfactory because the sleeving is bulky, and thus takes up an unacceptable percentage of the volume available for the electrical conductor in many electrical devices, thereby adversely affecting the power density which can be achieved. The sleeving also has a propensity, in some applications, to fray and create undesirable dust, and to act as a wick for moisture.
The second basic approach used in the past encompasses a number of schemes for utilizing combinations of organic and inorganic coatings or potting materials to provide an electrical insulation system. In some instances, an initial layer of an organic polymer is applied to the conductor, followed by a second layer of inorganic material. The polymer layer allows the insulated wire to be readily bent into the desired shape, with the inorganic layer being applied as a slurry either during or subsequent to forming the winding. The finished winding is then subjected to a high temperature in a controlled atmosphere to pyrolize the organic material and cure the inorganic material. In other instances a length of wire having either a woven or solid coating of cured inorganic material is bent to form the winding. A layer of organic material is then applied over the inorganic coating to fill any cracks in the inorganic coating. The organic layer is then often pyrolized, so that it does not further degrade or outgas when exposed to high temperature during operation of the winding.
There are two major problems with this second basic approach. First, the winding, and in some instances also the electrical device itself, must be heated to temperatures in the range of 700.degree. to 2000.degree. F. to cure the inorganic coating, and to pyrolize the organic coatings. Such heating can be detrimental to the electrical device, and adds additional fabrication time and expense. The second problem is that a carbon residue is left following pyrolysis. Carbon is electrically conductive. The residue thus degrades the dielectric strength of the insulation. To remove the carbon residue, some approaches heat the completed winding or electrical device in an oxidizing environment to temperatures in the range of 700.degree. to 2000.degree. F. in order to volatilize the carbon and remove it from the insulation in the form of CO or CO.sub.2 gas. Although the volatilization process may remove much of the carbon, the cost and undesirable side effects incident with subjecting the winding or electrical device to high temperatures remain. In addition, some electrical devices include components made from materials which cannot withstand exposure to an oxidizing environment.
In one variant of the second basic approach, the electrical conductor is initially coated with a mixture of an inorganic material suspended in an organic binder. The mixture may also contain an organic solvent to facilitate application of the coating onto the conductor. The coated wire is then subjected to a heating cycle to partially cure the coating by driving off the solvent. The winding is then formed by bending the wire with the partially cured coating about a mandrel, or other form. The completed winding is then subjected to the pyrolizing and volatilizing steps previously described in order to remove the organic binder and produce an essentially inorganic coating on the electrical conductor. Although this approach would appear to alleviate some of the propensity of pure inorganic coatings to crack during bending and use, experience of the instant inventors has shown that cracking is not completely eliminated for small radius bends. The previously described problems of increased cost and risk of damage to the electrical device are also not alleviated with this approach.
In summary, the prior art does not teach a completely satisfactory high temperature electrical insulation system for use with windings in electrical devices. Existing systems are compromised by the potential for low dielectric characteristics due to the presence of cracking or carbon traces in the coating material. The need for exposure of the winding or the completed electrical device to temperatures as high as 2000.degree. F. in order to pyrolize or volatilize the organics creates unwanted cycle time and cost, as well as undesirable risk of damage to the device.
Accordingly it is a primary object of our invention to provide an improved high temperature insulation system for a winding of an electrical device, and methods for fabricating such a system. Other objects include providing:
1. an insulation system which does not include organic components for operation or in its manufacture; PA1 2. an insulation system which allows the use of commercially available pre-insulated inorganically coated wire; PA1 3. an insulation system which does not require application of heat for curing any of the constituent coatings or other elements; PA1 4. an insulation system that is applicable to a wide variety of electrical devices; and PA1 5. an inexpensive arrangement for efficiently producing insulated electrical windings for use at elevated temperatures.